Physicist; b. at Broglie near Bernay, Normandy, 10 May, 1788; d. at Ville d'Avray, near Paris, 14 July, 1827. His early progress in letters was slow though he showed while still young an aptitude for physical science. In his seventeenth year he entered the Ecole Polytechnique in Paris where he attracted the attention of Legendre. After spending some time at the Ecole des Ponts et Chaussées he was assigned to the engineering corps and served successively in the departments of Vendée, Drôme, and Ille-et-Vilaine. He lost his appointment through politics on the return of Napoleon from Elba. In 1819 he was made a member of the Lighthouse Commission, becoming its secretary in 1824, and was an examiner at the Ecole Polytechnique from 1821 to 1824. Shortly afterward his health, which had never been robust, became so weakened that he was obliged to give up nearly all active work. He was unanimously elected a member of the Académie des Sciences in 1823, and in 1825 was made an associate of the London Royal Society, receiving its Rumford Medal on his death-bed.

Fresnel occupies a prominent place among the French physicists of the nineteenth century. His chosen field of research was optics, and in a series of brilliant memoirs he did much to place the wave theory upon a firm basis. He introduced with conspicuous success the conjecture of Hooke (1672) that the light vibrations are transverse. His first paper was on aberration, but it was never published. In connexion with his study of the theory and phenomena of diffraction and interference he devised his double mirrors and biprism in order to obtain two sources of light independent of apertures or the edges of opaque obstacles. His article on diffraction won the prize of the Académie des Sciences in 1819. He extended the work of Huyghens and others on double refraction and developed the well-known theory which bears his name. With Arago he investigated the phenomena and formulated the laws of the interference of polarized light. He showed how to obtain and detect circularly polarized light by means of his rhomb. An account of his more important contributions to optics may be found in Preston's "Theory of Light" (New York, 1891), or Wood's "Physical Optics" (New York, 1905). Fresnel gave a course of physics for some months at the Athenée in 1819, but otherwise had no academic connexions apart from his position as examiner at the Ecole Polytechnique. Mots of his researches were carried on in the leisure he could obtain from his professional duties. In applied optics mention should be made of his system of lenses developed during his connexion with the lighthouse commission which has revolutionized lighthouse illumination throughout the world. Fresnel was a deeply religious man and remarkable for his keen sense of duty. A three-volume edition of his complete works was published in 1866.

ARAGO, Oeuvres Complètes (Paris, 1854), I, 107-185; VERDET,Oeuvres Complètes d'Aug. Fresnel, introduction in vol. I (Paris, 1866); HELLER, Geschichte der Physik (Stuttgart, 1884), II.

H. M. BROCK